Phase-splitter

ABSTRACT

First and second transistors are connected at their bases to share a bias current supplied to them from a constant current generator. The emitter electrodes of the first and second transistors are connected respectively to the input terminal and to the output terminal of a current mirror amplifier. Input signal applied to the input terminal of the current mirror amplifier causes push-pull variations in the collector currents of the first and second transistors.

The present invention relates to phase-splitters--that is, to circuits providing push-pull output signals responsive to an applied input signal.

In a phase-splitter embodying the present invention first and second transistors are connected at their base electrodes to share a bias current supplied to them from a constant current generator. The emitter electrodes of the first and second transistors are connected respectively to the input terminal and to the output terminal of a current mirror amplifier. Input signal applied to the input terminal of the current mirror amplifier causes pushpull variations in the collector currents of the first and second transistors.

In the drawing:

FIGS. 1 and 3 are schematic diagrams of phase splitters which are representative embodiments of the present invention, and

FIG. 2 is a prior art constant current generator useful in combination with the FIG. 1 phase-splitter for biasing it advantageously.

In FIG. 1, a constant current generator S₁ supplies a bias current to the joined base electrodes of transistors Q₁ and Q₂. (The term "constant current generator" as used herein, refers to a circuit whose output current does not change appreciably with variation in the load to which this current in supplied, although this current may vary as a function of other parameters of the generator, such as its temperature or the common-emitter current gain β of its transistors.) Transistors Q₃ and Q₄ are connected as a current mirror amplifier CMA₁ having an input terminal 11, a common terminal 12, and an output terminal 13.

CMA₁ is characterized by a current gain between its terminals 11 and 13 substantially equal to -1 times the transconductance of Q₄ divided by the transconductance of Q₃. (The relative transconductance of transistors with similar junction profiles, as well known, is determined by the ratio between the effective areas of their respective base-emitter junctions.) This current gain, usually made to be -1, in phase-splitters embodying the invention, by matching the effective base-emitter junction areas of Q₃ and Q₄ determines the ratio between the quiescent emitter currents of Q₁ and Q₂. Assuming CMA₁ to have such a current gain of -1 and Q₁ and Q₂ to have similar common-emitter current gains of β, the quiescent emitter currents of Q₁ and Q₂ are essentially equal, and the bias current from S₁ splits evenly between the base electrodes of Q₁ and Q₂ to support these quiescent emitter currents. If these quiescent emitter currents are assumed to have a normalized value of unity, S₁ supplies a bias current equal to 2/(β+1).

A source S₂ of signal current applies that signal current to the interconnection of the emitter electrode of Q₁ to input terminal 11 of CMA₁. This application is shown as being via capacitor C₁, but direct coupled arrangements for applying signal may be employed instead.

Signal from S₂ perturbs the quiescent current flow into the input terminal 11 of CMA₁ with normalized value of unity, by an amount Δ. The (1+Δ) current into CMA₁ via terminal 11 gives rise to a current (1+Δ) into CMA₁ via terminal 13. This causes Q₄ to demand a corresponding output current (1+Δ) from Q₂, causing the base current demand of Q₂ to be (1+Δ)/(β+1). Subtracting the base current of Q₂ from the 2/(β+1) current supplied by S₁, leaves a base current of (1-Δ)/(β+1) for Q₁. The emitter current of Q₁ is thus (1-Δ), and current generator S₁ must supply a signal current of 2Δ to support the assumed signal conditions. Consequently, the input impedance of the FIG. 1 phase-splitter configuration is one-half that of CMA₁ by itself. The input impedance of CMA₁ is substantially that afforded by self-biased transistor Q₃ known to have a value of one divided by 25 millimhos per milliampere of emitter current.

The collector electrodes of Q₁ and Q₂ are biased to condition them for normal transistor action; and the (1+Δ) and (1-Δ) emitter currents of transistors Q₁ and Q₂, respectively, are amplified by the common-base amplifier actions of these transistors to develop β(1+Δ)/(β+1) and β(1-Δ)/(β+1) respective collector currents. These collector currents with similar quiescent components and push-pull signal components provide the output signals of the FIG. 1 phase-splitter and may, as shown, be applied to resistive loads R₁ and R₂ to convert them from current to voltage form. The push-pull output signal voltages are then available at terminals 14 and 15.

It is important to the understanding of the present invention to appreciate that the interconnection between the base electrodes of Q₁ and Q₂ is a bidirectional coupling means permitting the exchange between them of the push-pull variations in their base currents responsive to input signal applied by S₂. It is this bidirectional coupling means, which may take other known forms, that removes the need for any other signal path to the base electrodes of Q₁ and Q₂ and facilitates using a constant current generator S₁ to supply quiescent base currents to Q₁ and Q₂. The current generator S₁ is made to have a relatively high source impedance which does not interfere with this exchange of base current variations between Q₁ and Q₂.

Δ may assume positive or negative values. Δ may exceed unity amplitude to obtain Class B phase-splitter operation if precautions are taken to clamp the base potentials of Q₁ and Q₂ under such large-swing conditions from swinging below about 1 volt. This, to avoid saturation of Q₄ and biasing of the collector of Q₃ below ground.

CMA₁ can be replaced with a common-emitter amplifier transistor such as Q₄ alone. This will cause the signal at terminal 14 to be smaller than that at terminal 15 by the current gain β of Q₄, which can be compensated for by a subsequent amplifier. Alternatively, the β of Q₄ can be reduced to unity amplitude by known manufacturing techniques--e.g., using large base widths and high base region doping levels as compared to conventional manufacture.

FIG. 2 shown a constant current generator of the type described in U.S. Pat. No. 3,891,935, which may be employed as the generator S₁ of the FIG. 1 phase-splitter. Transistors Q₅ and Q₆ are in Darlington cascade arrangement with a direct-coupled degenerative feedback connection of the collector electrode of Q₆ to the base electrode of Q₅. This feedback regulates the potential at the base electrode of Q₅ to the sum of the base-emitter offset potentials V_(BEQ5) and V_(BEQ6) of Q₅ and Q₆, respectively. In this regulation process, the collector current of Q₅ is adjusted to a value substantially equal to: (+V_(CC) -V_(BEQ5) -V_(BEQ6)), the potential drop across R₃, divided by the resistance R₃. The base current of Q₅, smaller by a factor β, is coupled by the common-base amplifier action of Q₅ to the input terminal 16 of a current mirror amplifier CMA₂ of complementary conductivity type of Q₁, Q₂,Q₃, Q₄, Q₅, Q₆. CMA₂ provides an oppositely directed current from its output terminal 18 for application to the joined base electrodes of Q₁ and Q₂. The supply provides a current inversely proportional to β, that substantially cancels out the effect of variation in the β of Q₁ and Q₂ upon the potential drops across R₁ and R₂. If CMA₂ has a current gain of -1 between its terminals 16 and 18, making R₃ = R₁ ≈ R₂ will bias terminals 14 and 15 one base-emitter potential offset above +V_(CC) /2. Emitter follower transistors can receive their input signals from terminals 14 and 15 to supply push-pull output signals superimposed on +V_(CC) /2, facilitating direct coupling to succeeding stages.

FIG. 3 illustrates a second embodiment of the invention. It includes the elements of FIG. 1 and, in addition: (a) Q₁ has another transistor Q₁₁ connected with it to form a current mirror amplifier CMA₃ with input, common and output terminals 31, 32 and 33, respectively, and (b) Q₂ has another transistor Q₁₂ connected with it to form a current mirror amplifier CMA₄ with input, common and output terminals 41, 42 and 43, respectively. While the current gains of CMA₃ and CMA₄ can be made -1 as between their respective input and output terminals, it is usually desirable to make these gains larger by factors approaching the β's of their component transistors. This is done by making the effective areas of the base-emitter junctions of Q₁ and Q₂ larger than those of the base-emitter junctions of Q₁₁ and Q₁₂ by these factors connecting Q₁₁ and Q₁₂ to modify the effective current gains of Q₁ and Q₂ so as to be substantially independent of β means that the constant current source S₁ ' can supply β-independent bias current to the base electrodes of Q₁ and Q₂ without incurring β-related variations in the potential drops across R₁ and R₂. So, S₁ may simply consist of a resistance equal to R₁ and to R₂, connected between +V_(CC) and the interconnection of terminals 21 and 31 to bias terminals 14 and 15 one base-emitter potential drop more positive than +V_(CC) /2, when the gains of CMA₃ and CMA₄ equal -1.

One skilled in the art of electronic circuit design will appreciate that many other variations of the present invention exist. The current mirror amplifier CMA₁ of the particular type shown can be replaced by other known types, for example. CMA₃ and CMA₄ can be replaced by different types of current mirror amplifier or by other current amplifiers with well-defined current gains. A great variety of output loads may replace R₁ and R₂ and the constant current generators S₁ and S₁ ' may take other forms to suit those loads. 

What is claimed is:
 1. The combination of:a current mirror amplifier having an input terminal, an output terminal, and a common terminal, connected at its common terminal to a point at a reference potential; first and second loads; a terminal for an operating voltage; first and second transistors of the same conductivity type, each having base and emitter and collector electrodes; a connection of the emitter electrode of said first transistor to the input terminal of said current mirror amplifier; a connection of the collector electrode of said first transistor through said first load to said terminal for said operating voltage; a connection of the emitter electrode of said second transistor to the output terminal of said current mirror amplifier; a connection of the collector electrode of said second transistor through said second load to said terminal for said operating voltage; current generator means coupled to the base electrodes of said first and said second transistors for supplying them quiescent base currents; bidirectional coupling means between the base electrodes of said first and said second transistors for exchanging their respective base current variations between them; and means for applying an input signal to the connection of the emitter electrode of said first transistor and the input terminal of said current mirror amplifier.
 2. The combination set forth in claim 1 wherein said bidirectional coupling means between the base electrodes of said first and said second transistors consists of a direct connection between the base electrodes of said first and said second transistors, the sole other connection to the base electrodes of said first and said second transistors being that of said current generator means.
 3. The combination set forth in claim 2 wherein said current generator means provides a current inversely proportional to the common-emitter forward current gain of said first and said second transistors, thereby adjusting the quiescent collector currents of said first and second transistors to be substantially independent of the common-emitter forward current gains of said first and said second transistors.
 4. The combination set forth in claim 1 having:third and fourth transistors of said same conductivity type, having respective base and emitter and collector electrodes, said third transistor being connected emitter-to-emitter with said first transistor and having its collector electrode direct coupled to an interconnection between the base electrodes of said first and said third transistors, said fourth transistor being connected emitter-to-emitter with said second transistor and having its collector electrode direct coupled to an interconnection between the base electrodes of said second and said fourth transistors.
 5. A phase-splitter circuit comprising:a current mirror amplifier having input, common and output terminals; first and second transistors of the same conductivity type, each having base and emitter and collector electrodes; an interconnection of the emitter electrode of said first transistor and the input terminal of said current mirror amplifier for receiving an input signal; an interconnection of the emitter electrode of said second transistor to the output terminal of said current mirror amplifier; a constant current generator connected between the common terminal of said current mirror amplifier and an interconnection between the base electrodes of said first and said second transistors; means for biasing the collector electrodes of each of said first and second transistors relative to said common terminal to condition said first and second transistors for normal transistor action; and a pair of terminals to which the collector electrodes of said first and said second transistors are connected for supplying push-pull output signals responsive to said input signal.
 6. In combination:first, second and third current mirror amplifiers each having input, output and common terminals; an interconnection of the input terminal of said first current mirror amplifier and the common terminal of said second current mirror amplifier, for receiving an input signal; an interconnection of the output terminal of said first current mirror amplifier to the common terminal of said third current mirror amplifier; and a constant current generator connected between the common terminal of said first current mirror amplifier and an interconnection of the input terminals of said second and said third current mirror amplifiers, whereby push-pull output signals are afforded at the output terminal of said second and said third current amplifiers responsive to said input signal.
 7. In combination:first and second loads; first and second terminals for application of operating potential therebetween; first, second and third transistors of the same conductivity type, each having base and emitter and collector electrodes; connections of the emitter electrodes of said first, said second, and said third transistors respectively to said third transistor base electrode, to said third transistor collector electrode, and to said first terminal; connections of the collector electrodes of said first and said second transistors to said second terminal respectively through said first load and through said second load; current generator means coupled to the base electrodes of said first and said second transistors for supplying them quiescent base currents; bidirectional coupling means between the base electrodes of said first and said second transistors for exchanging their respective base current variations between them; and means for applying an input signal to the connecton of the emitter electrode of said first transistor to the base electrode of said third transistor. 